So far, an exhaust passage of an engine loaded on a vehicle has been provided with an exhaust purification catalyst, such as a three-way catalyst, with the aim of purifying substances to be discharged in association with combustion, for example, HC (hydrocarbons), CO (carbon monoxide), and NOx (nitrogen oxides). This exhaust purification catalyst has the functions of absorbing oxygen (O2) when in a lean atmosphere (oxidizing atmosphere) to accelerate the reduction reaction of NOx, and releasing the absorbed O2 when in a rich atmosphere (reducing atmosphere) to accelerate the oxidation reaction of HC, CO or the like.
From the points of view of decreasing fuel consumption in the engine or protecting the exhaust purification catalyst, fuel supply is stopped (so-called fuel cut) while a vehicle is traveling, for example, when the vehicle is decelerating. During such fuel cut, substantially air alone is discharged into the exhaust passage, so that a large amount of oxygen is supplied to the exhaust purification catalyst and absorbed thereby. If fuel supply is restored (fuel restoration) in this state, NOx cannot be reductively treated sufficiently by the exhaust purification catalyst, with the result that an exhaust gas containing large amounts of NOx is likely to be emitted into the atmosphere.
To solve such problems, there are, for example, known technologies which, when fuel supply is restored, exercise control for temporarily making a fuel amount increasing correction in accordance with the length of the period of fuel cut to bring the air-fuel ratio to a rich side as compared with a stoichiometric air-fuel ratio (enrichment control) (see, for example, Patent Documents 1 and 2). These technologies can desorb oxygen stored in the exhaust purification catalyst (O2 purge) and cut down on the amounts of NOx emissions into the atmosphere.